The present invention relates to a method for producing nickel (II) hydroxide from a powdered metallic nickel.
Nickel hydroxide is used for various industrial applications such as positive electrodes of alkaline storage batteries. This nickel hydroxide has been produced, for example, by adding sodium hydroxide and the like to a solution containing a nickel salt such as nickel sulfate, nickel chloride or nickel nitrate for alkalizing the solution and precipitating nickel hydroxide.
Such production method of nickel hydroxide has only a simple mechanism but requires a number of steps for carrying out the method. The use of sodium hydroxide for alkalizing a solution in a reaction vessel, particularly, produces sodium nitrate, sodium sulfate and the like as by-product, thereby necessitating removal of these substances in the form of a waste water out of a reaction system.
In order to solve this problem, a method in which nickel hydroxide is produced by dissolving nickel into an aqueous solution containing ammonium and/or an ammonium salt is disclosed in Japanese Laid-open Patent Publication Hei No. 8-34618.
According to this method, after the potential of the aqueous solution dissolving nickel reaches a reduction level, conversion of nickel into nickel hydroxide is facilitated by adding oxygen to precipitate nickel hydroxide.
The followings are reaction formulae representing generation of nickel hydroxide in this production method:
Ni+{fraction (1/20)}2+2NH4OHxe2x86x92Ni(NH3)22++OHxe2x88x92+H2Oxe2x80x83xe2x80x83(1)
Ni(NH3)22++xNH3xe2x86x92Ni(NH3)n2+(n=2+x)xe2x80x83xe2x80x83(2)
Ni(NH3)n2++2OHxe2x88x92xe2x86x92Ni(OH)2+nNH3xe2x80x83xe2x80x83(3)
Next, this method will be briefly explained, referring to each process in due order. (i) Addition and dissolution of a powdered nickel into an aqueous solution containing an ammonium ion
In this step, a powdered nickel is added to the above-mentioned aqueous solution containing ammonium for activation and dissolution.
The aqueous solution containing an ammonium ion used here is a solution, which contains a free ammonium ion and can dissolve metallic nickel, so the solution may be used in this step contains at least an ammonium salt capable of supplying a free ammonium ion. As the ammonium salt, there are, for example, ammonium sulfate, ammonium acetate, ammonium chloride, ammonium formate and ammonium phosphate, which may be used alone or in an arbitrary combination of two or more. Among them, ammonium sulfate and ammonium acetate are preferably used because they are able to facilitate dissolution reaction.
Also, since reaction between ammonium salt and ammonium is reversible, the above-mentioned aqueous solution containing ammonium may only include a relatively low concentration of an ammonium salt. (ii) Supply of oxygen and application of negative potential to the above-mentioned aqueous solution
In this step, a negative potential is given to the above-mentioned aqueous solution obtained by adding and dissolving the powdered nickel into the aqueous solution containing an ammonium ion, so that the potential of the aqueous solution can reach a reduction level to facilitate generation of nickel hydroxide upon supply of oxygen in the next step.
The potential of the aqueous solution to be applied may be xe2x88x92600 to xe2x88x92100 mV in consideration of the balance of Ni, O2, NH3 and H2SO4. Further, it is preferably xe2x88x92500 to xe2x88x92200 mV.
The temperature of the aqueous solution may be 20xc2x0 C. or higher and simultaneously lower than the boiling point thereof, because lower temperatures than this range would lead to lower yields. Further, it is preferably 30xc2x0 C. or higher and simultaneously lower than the boiling point thereof. In view of the energy efficiency, particularly, it is preferably 50 to 60xc2x0 C.
The pH of the aqueous solution may be 8.5 to 12 in view of the generation efficiency of nickel hydroxide, and it is preferably 9 to 12. According to the prior art techniques, such pH of the aqueous solution is controlled by adjusting an amount of supplied oxygen and powdered nickel as a raw material.
Such method of producing nickel hydroxide, however, has problems in that the amount of supplied oxygen and nickel must be adjusted in order to control pH of the solution and that physical control in terms of crystallinity, particle size, tap density and the like is difficult.
In view of the foregoing, the present invention intends to provide a method for producing nickel hydroxide which is capable of an easy pH control without substantially yielding waste water.
The method for producing nickel hydroxide in accordance with the present invention comprises the steps of: adding and dissolving a powdered nickel into an aqueous solution containing an ammonium ion; and controlling pH of the aqueous solution by supplying a hydroxide ion generated by water electrolysis thereto to produce nickel hydroxide.
In this case, it is preferable to capture a hydrogen ion generated by the electrolysis by a cation exchange membrane.
Further, an amount of supplied hydroxide ion generated by the electrolysis can be controlled by an anion exchange membrane or an amount of current flowing in the electrolysis.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawing.